Fix: Serialize session creation to prevent permission-denied race condition

[jrob5756]

Summary

Serialize create_session calls in CopilotProvider so that concurrent for-each agents don't hit the race condition described in #27.

Problem

The copilot-sdk's CopilotClient.create_session() has a race window: the CLI starts using a session before the Python SDK registers it in _sessions. When the CLI sends a permission.request for the not-yet-registered session, the SDK raises ValueError("unknown session"), which the CLI surfaces as "Permission denied" to the model. The agent then retries every tool for up to 30 minutes before timing out.

This only happens when multiple create_session calls overlap — i.e., in for_each groups with max_concurrent > 1.

Proposed Fix

Add a lock around create_session in CopilotProvider.execute() so sessions are created one at a time. Sessions still run in parallel — only creation is serialized.

class CopilotProvider:
    def __init__(self, ...):
        ...
        self._session_create_lock = asyncio.Lock()

    async def execute(self, ...):
        ...
        async with self._session_create_lock:
            session = await self._client.create_session(session_config)
        ...

Cost: ~200ms per session creation × N sessions, which is negligible against workflow runtimes of 10+ minutes.

Benefit: Eliminates the race window entirely. Each session is fully registered in _sessions with its permission handler before the next create_session begins, so no permission.request can arrive for an unknown session.

Related

• Race condition in SDK causes Permission denied in for-each groups under concurrency #27 — Root cause analysis of the race condition